The present invention relates to fuel cells. More specifically, the present invention relates to a fuel cell apparatus having an electrolyte disposed between a fuel electrode and an oxygen electrode, wherein a fuel, such as hydrogen, as well as air are fed to the fuel cell to generate desired electromotive force, and a method for controlling the fuel cell.
In general a fuel cell is an apparatus in which an electricity generator generates electric power by supplying fuel fluid, such as hydrogen gas or methanol, and a solid polymer type fuel cell generally has a structure such that a proton conductor membrane is disposed between an oxygen-side electrode and a fuel-side electrode. Air is fed to the oxygen-side electrode for supplying oxygen thereto, and fuel fluid is supplied to the fuel-side electrode. When the fuel cell generates electricity, protons move an electrolyte membrane which is an ion-exchange membrane, and reacts with oxygen on the oxygen-side electrode to cause a current and form water on the oxygen-side electrode. The electricity generator portion of the fuel cell is called electrolyte membrane-electrode composite or membrane and electrode assembly (MEA), and a fuel cell having a plane structure is constituted by the electrolyte membrane-electrode composite solely or the composites arranged in a plane, or a fuel cell having a stack structure is constituted by the composites stacked on one another.
Recently, in the field of vehicles for transport and the like, application of the fuel cell to electric car or hybrid car is intensively expected, and, in addition, the fuel cell is expected to be put into practical use as a household power supply system and the like. Further, taking advantage of the lightweight and small fuel cell, studies and development are being made on application of the fuel cell to portable apparatus, small-size power supply, and the like.
As a fuel cell of one type, there is a fuel cell having no moistening device for keeping humidity of the electrolyte membrane or the like (hereinafter, the fuel cell of this type is referred to as “self-moistening type fuel cell”). The self-moistening type fuel cell is constructed so that moisture formed on the oxygen-side electrode causes the electrolyte membrane to be wet to promote ion-exchange. In the fuel cell, control of evaporation of the moisture formed leads to control of the electricity generation performance of the fuel cell, and the output voltage directly affects heat generation and the output current directly affects water formed. Therefore, it is necessary to operate the self-moistening type fuel cell so that the electrolyte membrane is appropriately wet by utilizing formed water, which is directly affected by the output current, and excess water formed would not block the feed passage of oxygen.
However, especially in the above-mentioned self-moistening type fuel cell, when the load current is decreased or the air feed rate is increased during the operation, the electrolyte membrane is reduced in moisture and dried. In the fuel cell dried, the ion-exchange characteristics of the electrolyte membrane become poor, so that the output of the fuel cell is considerably lowered. In addition, not only when the load current is decreased during the operation, but also, for example, when the fuel cell is started again after the fuel cell is allowed to stand for a long time, the electrolyte membrane is in a dry state, and it is not easy to let the electrolyte membrane be wet again after the start, and hence it takes a period of time as long as several days to recover the original performance of the fuel cell to obtain a desired rated output. The problem of drying of the electrolyte membrane is marked especially in an open-air type fuel cell in which no compressed air feeding is conducted, and the problem of drying arises merely if the fuel cell is allowed to stand after the operation, causing a disadvantage in that the output characteristics are lowered in a short time.
A need therefore exists to provide improved fuel cells and methods of producing and using same.